128 DYNAMICAL GEOLOGY. 



affirmed to be modern ; for they have probably been in progress ever since 

 the land first emerged from the ocean so that air and water could begin the 

 work. 



In the destruction of the iron-bearing minerals of surface rocks, the iron 

 oxide combined with a humus acid is often carried into marshes to make 

 '' bog iron ores." The ores thus formed have much value, although likely 

 to contain phosphates as impurity, because of the animal and vegetable mat- 

 ters that live and die, or find burial, in swamps. 



The consolidation of beds of sand and gravel, or layers of rock, is another 

 of the constructive effects of the iron oxide that is distributed through the 

 material of the beds. In the simplest form of it, the waters, filtering 

 through soil and gravel, take up enough oxide of iron to cement a bed of 

 pebbles lying at a lower level on another layer sufficiently close in texture 

 to hold the water and give the iron a chance to deposit; and this is one 

 way in which what is called hard-pan is sometimes made. The underlying 

 impervious bed is not absolutely necessary to the result, although promoting 

 it. The pebbles wet with the ferruginous waters, when they dry in times 

 of drought, take a deposit of iron ; and this process may end in complete 

 consolidation. In other cases the oxide is produced throughout the deposit 

 under the action of infiltrating waters, and slowly becomes a cement as it 

 solidifies. 



This mode of consolidation without aid of heat is not the most common 

 nor the most efficient. 



The beds of sulphur of the world have been made by the two processes 

 mentioned on page 125, and chiefly the former. 



Hydration, or the Chemical Absorption of Water. 



Many minerals take up water on "weathering." But this usually is 

 an accompaniment of commencing decomposition. An example of simple 

 hydration of geological importance is the change of anhydrite (CaO.SOa) to 

 gypsum (CaO.SO3.2H2O). As the minerals are very unlike in cleavage, and 

 both occur in large beds, the change is strikingly noticeable. 



Carbonic Acid, Humus Acids. 



1. General action. — Carbonic acid (CO2) is ever present in the atmos- 

 phere, of which it constitutes 3 parts in 10,000 by volume, and in all rain 

 water, river water, and sea water. It is often given off by mineral springs, 

 and occasionally escapes in large volumes from fissures in volcanic regions. 

 In the northeast corner of Yellowstone Park is " Death Gulch," where the 

 gas rises freely from the waters of Cache Creek, to the destruction of bears 

 and other wild animals. Butterflies and other insects, besides skeletons of 

 bears, elk, squirrels, etc., attest to its deadly character (W. H. Weed, 1889). 

 Death's Valley in Asia Minor, and the Dog's Grotto at the Solfatara near 

 Naples, are other localities of escaping carbonic acid. 



